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The Hopeless 350 MAG MPI

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toy4two

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I've become that guy, the one who is on a first name basis with my parts guy.

While I've been on every local mechanics waiting list going on 2 months now I'm trying to get back on the water with a working boat. The engine is giving no beeps but the Guardian Alarm beeper is connected and working. No codes in VesselView Mobile.

The only problem with my Mercruiser 350 MAG MPI with serial numebr 0M627854 is It will not go above 3000 RPM. It was fine just 2 months back and could hit 4800+. Running at 10MPH all day just isn't fun.

I started with the basic troubleshooting. First the fuel system was gone through from the tank to the high pressure fuel pump. The following were replaced:
low pressure fuel pump, high pressure fuel pump, inline fuel filter, fuel water seperator, fuel pressure regulator ( 43 psi ). Checked fuel pressure at idle about 37 psi, dead on for idle and pulilng the vacuum line it jumps to 43 psi. Screen under the FPR is new and clean, all fuel lines not kinked. Enzyme Ethanol fuel additive for 85 gallons added. I will take a fuel pressure reading tomorrow at 3000 RPM.

Still no change. Moved onto the ignition system and replaced corroded cap and rotor, plug wires, spark plug ( gapped .060 old spec ) and new ignition coil.

Running exactly the same. Now I've moved onto the sensors. I replaced IAC valve, new IAC muffler, and cleaned the flame arrestor. which was pretty clean as I have 185 hours on my 2013 engine. Also new Crankshaft Position Sensor (CPS), Throttle Position Sensor (TPS), cleaned the MAP sensor with alcohol (but new one on order of course!).

About the only things not replaced are the MAP sensor, fuel injectors, distributor, and ECU (2013 Mercury ECU created Feb 2013). All these minus the ECU are on order for arrival next week.

I don't know how it can go from perfect to 3000 RPM overnight, especially with all these new parts. I bought a Rinda scan tool but bought the Mercruiser Cable but actually need the CAN cable (2013) so still unable to read ECU parameters.

Does anyone have any guesses or experience with low WOT problem on 350 MAGs? :( I'm out of time and money on this one, even *IF* I heard back from a mechanic anytime soon, thank you in advance.
 
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I'll assume that you have eliminated the obvious like the throttle cable adjustment, fuel flow rate and any mechanical issues like something in the drive.
 
throttle cable adjustment is spot on, disconnected inspected and reconnected it today and went through the whole range of the throttle plate opening.

took it out to measure fuel pressure at the 3000 wall its hitting, it was about 42 psi but the gauge was bouncing around on top of the engine so close enough to 43 psi for me. At idle it dropped down to 37 psi. Doesn't look like fuel obstructions.

I also did a compression test and got the following values,

Cylinder PSI
8 175
6 165
4 168
7 173
5 165
3 160
1 168

Would the drive fail in such a way where it limits power to 3000 RPMs. I've been using the boat at this RPM for 2 months now and it hasn't gotten any worse. Boat sits in a slip so no real way to inspect the drive.

On a 2013 PCM-09 ECU Mercruiser would there be some "service mode" that it gets stuck in from having a Guardian alarm set becuase I had a sensor disconnected and it won't clear unless a scan tool computer is used to clear it? This just seems like the most logical thing since I've tested, retested and redone the entire fuel and ignition systems and all the sensors I can think of that the ECU uses at high RPM (my engine doesn't use a cam sensor in the distributor). My guradian alarm beeps once at key on and I've disocnnected the IAC and gotten it to beep twice.

For S+G today I also put on yet another new CAP and ROTOR this time a Napa Echlin top of the line, and put in yet another new set for spark plugs, this time NGKs instead of AC Delco yet nothing has changed. I can't wait until the CAN adapter arrives from Rinda as i want to see the temperature the ECU thinks it is running at (170 on the dash) and also how the timing is advancing, lastly record then erase all code history.
 
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Chris might be on to something there. I was thinking that the thing was getting fuel starved (yes even tho it says it has pressure, it may not be flowing enough) or that there might be a rotten bearing or something the engine was working against. Def need to see the log.
 
o2batsea said:
Chris might be on to something there. I was thinking that the thing was getting fuel starved (yes even tho it says it has pressure, it may not be flowing enough) or that there might be a rotten bearing or something the engine was working against. Def need to see the log.
Click to expand...

The saga continues.

Went to the boat yesterday as the Rinda CAN adapters arrived. Unplugged my VesselView Mobile Bluetooth module from the CAN port and plugged in the TechMate Pro. Unit powers up I select sterndrive/inboard then Starboard then it just says cannot find ecm please turn key at which point I’ve tried key off, key on and engine running to no avail.

So so with no ability to scan the engine I put on a new gear switch sensor and also a new starting battery as my other one was weak. Took it for a spin and same issue 3000 RPM max.

is there a temperature sensor on the engine the ecu uses to determine it’s too cold to allow over 3000 rpm or alternatively do 2013 engines have some service mode it’s stuck in?
 
talked to Rinda, apparently the boat can have multiple diagnostic ports, the 10 pin CAN adapter I plug in my VesselView Mobile into is the wrong one, there should also be a 4 port plug somewhere else, possibly used by the depth finder by the previous owner. Engine is a rebuild from Mercury so likely has an 555 ECU not PCM-09 like you would expect for a 2013. back to the boat tomorrow.
 
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Success I found the four pin diagnostic port and can now see the ecm values.

one that is sticking out is BLOCK PRESSURE which is 0! Is this normal?
 
Looking at the Freeze Frame fault history it always reads 0 for Block Pressure.

Finally had a mechanic call me back, its only been a month, they do exist. He says its the spark plug wires in the wrong order or bad wire. Does anyone have a diagram of the number of each cylinder so I can redo them? He said he sees this all the time if 350 Mag from 2013 won't go over 3000 RPM and you smell fuel its 9 times out of 10 the wires are on the wrong cylinders (or the distributor cap/rotor) I've been through 3 caps and 3 sets of plugs, and on my second set of wires, I'm guessing they are on the wrong plugs!
 
BTW the Rinda detects the throttle moving into gear. I also see the TPS move from 0% to 100% exactly matching the throttle lever.
 
I think it has something to do with the cooling system as now the boat shuts down and gives me alarms about 3 - Sea water pressure or something. I've just replaced the impeller, thermostat, manifolds, risers, and saw half of my exhaust flapper is missing. I don't think there is anything more to replace on this boat.
 
tech came out and finally found the root caues. #1 the 3 month old water pressure sensor has failed #2 the engine trim button is REVERSED! I kid you not. This entire time I'm thinking I'm putting the trim a little bit more down only to be moving it up, no freaking wonder it could never get on plane.

At first he was saying welp bad news, needs a new drive coupler, the rubber coupler is giving an overspeed error meaning the boat engine is pushing all it can and not going anywhere with the drive. Engine out, drive off, and boat pulled $$$. As we were heading back in he thought the white water astern was too frothy for 5 MPH. Asked me to move aside and fiddled with the trim for a good 5 minutes up and down and up and down and up and down and discovered it was reversed!

He says lets turn around and re-test. We do and boom instantly on plane got it up to 3700 RPMs no sweat and had more in it but didn't need to see anymore and that lasted a good 30 seconds before Guardian kicked in for 0 water pressure at that RPM.

He will reverse the trim wiring, put in a new sensor and said to always keep one on board as they fail frequently.

Since I've owned the boat and this is my first, I've never really touched the engine trim but apparently someone did who I let drive it and its was never the same. Thanks all for the suggestions, this was truly one someone had to be on the boat to observe. Talk about a waste of money and time replacing everything imaginable when the drive trim was reversed.
 
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